As a form of optical recording media for audio and video and other optical recording media for recording various information, for example, an optical disc, on which information signals are written by embossed pits, is widely used. The most basic form of optical disc is shown in FIG. 12. This optical disc 2 comprises an information recording layer 8 having a reflective film 6 on a light transmission molded substrate (supporting plate) 4. A protective layer 12 made of an UV curable resin or the like is formed thereon. For this optical disc 2, a laser beam La is irradiated on the side of the molded substrate 4 acting as a light transmission layer so as to read an information signal from the information recording layer 8.
A recording density of the optical disc 2 can be increased as a spot diameter of the used laser beam La is reduced. The spot diameter is proportional to: λ/NA (λ: a wavelength of a laser beam, NA: a numerical aperture of an objective lens). Therefore, in order to reduce the spot diameter, it is necessary either to reduce the wavelength λ of the laser beam La or to increase the numerical aperture NA of the objective lens.
As the wavelength λ of the laser beam La, a wavelength of 780 nm is used for CDs, whereas a wavelength of 650 nm is used for DVDs. For the future, a wavelength in the vicinity of 400 nm corresponding to a blue laser is expected for use. The spot diameter is reduced by the amount of a decrease in wavelength, so that a recording density is increased.
On the other hand, coma aberration is increased as the numerical aperture NA is increased. In order to keep coma aberration small even with the increased numerical aperture NA, it is necessary to reduce a thickness of the light transmission layer (the molded substrate 4) through which the laser beam La passes.
Conventionally, as the light transmission layer (the molded substrate 4) of the optical disc 2, a molded product made of a resin formed by injection molding or the like is frequently used. By this fabrication method, however, it is difficult to fabricate the molded substrate 4 to have a smaller thickness than that of the currently used DVDs with good accuracy using the current technique. More specifically, in the optical disc 2 having a structure using the molded substrate 4 as a light transmission layer as shown in FIG. 12, a technique of increasing a recording density with a higher NA of the laser beam La to increase the capacity reaches the limit in the present state in view of the fabrication technique.
To cope with this, two approaches of improvement techniques have been proposed.
The first approach of improvement consists in increase in capacity by multiplication of the number of information recording layers.
For example, a DVD-9 has a double-layered information recording layer in a structure as shown in FIG. 13. More specifically, a first information recording layer 32 is formed on an upper surface of a first molded substrate (light transmission layer) 30 and then is covered with a translucent film (translucent reflective layer) 34. In the completely same manner, a second information recording layer 38 is also formed on an upper surface of a second molded substrate 36 (in the drawing, being oriented downward) and is then covered with a reflective film (reflective layer) 40. The first substrate 30 and the second substrate 36 are bonded with each other through transparent bonding means (light transmission layer) 42. By irradiation of the laser beam La, an information signal is read from the first and second information recording layers 32 and 38 through the first molded substrate 30. As a result, the capacity can be increased by increasing the number of information recording layers (doubling a substantial area of the information recording layers).
On the other hand, the second approach of improvement mainly consists in further increasing the NA of the laser beam to increase a recording density of information signals in the information recording layer, thereby increasing the capacity. For example, the related art is disclosed in Japanese Patent Laid-Open Publications Nos. 1997-235638, 2000-203724, and the like. Specifically, as shown in FIG. 14, a molded substrate 50, which does not required to transmit any light, is formed by injection molding to have a large thickness as a base of a disc body. Next, an information recording layer 54 having a reflective layer 52 is formed on the molded substrate 50. An extremely thin (at the maximum, about 300 μm) light transmission layer 56 is deposited thereon by means such as attachment of a light transmission sheet, resin coating through spin coating or the like.
The laser beam La is irradiated on the side of the thin light transmission layer 56 to record and/or read information on and/or from the information recording layer 54. In this manner, it is possible to form the thinner light transmission layer 56 while ensuring the entire strength on the side of the molded substrate 50. Therefore, a higher density (larger capacity) can be achieved by the approach of further increasing the NA of the laser beam.
Under the actual conditions, however, each of the above-described first and second approaches of improvement techniques still has various problems.
For example, in “multiplication of the number of information recording layers” corresponding to the above-described first approach of improvement, it is certain that substantial enlargement of the area of the information recording layers is made possible owing to multiplication of the layers. However, access to each of the information recording layers is achieved through the thick molded substrate 36 as in the conventional case. Therefore, the problem that it is hard to achieve a higher density for each layer still subsists. More specifically, capacity is expected to be increased only by the amount of increase in area achieved by multiplication of the layers.
On the other hand, for the approach of improvement that “density (capacity) for each layer is increased by increasing NA through a thin light transmission layer” corresponding to the second approach of improvement, various problems still remain in a concrete way to form thin light transmission layer.
Generally, as a method for forming the thin light transmission layer, a method of bonding a transparent sheet onto a molded substrate (or a supporting plate), a method of applying (coating) a resin by spin coating and the like have been proposed.
However, the method of bonding a transparent sheet is likely to have elevated fabrication cost because strict optical characteristics of the sheet itself and thickness accuracy are required. Moreover, even if the thickness accuracy of the transparent sheet is good, an uneven thickness or uneven application of an adhesive changes the optical characteristics to degrade the disc characteristics in some cases. Thus, high accuracy is also required for the application of an adhesive. Accordingly, under the actual conditions, many problems remain in this method.
The method of forming a thin light transmission layer by application of a resin through spin coating specifically consists in making a liquid UV curable resin (liquid to be applied) flow on a surface of a molded substrate (supporting plate) while rotating the molded substrate with a spindle so that the resin is uniformly extended in accordance with the rotation of the molded substrate to coat the resin layer. Thereafter, the coated resin is irradiated with ultraviolet rays so as to be cured. According to this method, the conditions such as a thickness of the coating can be adjusted by controlling a rotation speed of the spindle, application time and a viscosity of the application liquid.
However, this method has a problem in that the uniformity of the surface of the coating film is difficult to keep in such a way that a concentration (raise) of the application liquid is likely to occur, in particular, in the vicinity of the outer periphery. The raise does not become a serious problem in the case where there is a relatively large distance between a laser optical system and a disc such as a conventional CD or DVD. However, under the conditions where a gap between a laser optical system and a disc is reduced by a higher NA, it becomes an unignorable obstacle.
In the case where a relatively thick (for example, 100 μm) coating layer is formed, in particular, a thickness or a radial width of the raise becomes remarkably large to penetrate into an information recording area in some cases. Therefore, there is a possibility that a recordable area may be decreased to impair the recording/reading of information signals if no countermeasure is taken. Moreover, it also causes the collision of a laser optical system against a disc in some cases.
Regarding a treatment of the raise or the maintenance of smoothness of the coating surface, various proposals for ways of coping with them have been made in many documents, for example, Japanese Patent Laid-Open Publications Nos. 1999-203724, 1999-86355, 1999-86356, and the like. However, this signifies, in other words, “some measure” is necessarily needed irrespective of a concrete method thereof. Correspondingly, a fabrication process is complicated while cost is increased.